Marina Litvinenko, the widow of the poisoned Russian spy Alexander Litvinenko, has made a plea for public donations to continue her legal fight to discover the truth about her husband's murder.

In a tearful interview with the Guardian, Litvinenko said she and her son risked losing their home and any source of income if she failed in her attempt to force a public inquiry into the 2006 killing – the only mechanism by which evidence of the Russian state's culpability can legally be considered.

"What I am doing I am doing for my husband," she said. "But if I decide to carry on [with my legal fight], there is a huge risk that we will be homeless and will lose everything. I need to take some very serious decisions."

Litvinenko is trying to overturn home secretary Theresa May's decision in July to block a public inquiry – a refusal partly motivated, May admitted, by a fear of offending Russia. But a panel of judges on Thursday declined to protect the dead man's widow with a cost-capping order for the government's legal bills if she loses her legal bid.

The ruling means she would be liable for costs of £40,000 for government lawyers if she continues to push for a judicial review but loses. Her own legal team is working pro bono.

The government had argued that she had the means to meet any potential legal bills if she lost and says it has not ruled out the possibility of a public inquiry after the inquest.

Alexander Litvinenko. Photograph: Alistair Fuller/AP

Alexander Litvinenko, a former KGB agent who had been granted political asylum and become a British citizen, died in November 2006 after drinking a cup of tea that had been laced with radioactive polonium.

Two Russian men have been named as prime suspects in the murder, and legal hearings preceding the dead man's inquest have heard that the government has evidence amounting to a prima facie case that he was killed on the orders of the Russian government.

The coroner, Sir Robert Owen, has also been told that Litvinenko was working as a paid MI6 spy at the time of his death.

Owen wrote to the government earlier this year saying that a public inquiry was the only way he could mount a proper investigation into the killing. If forced to examine the death only by means of an inquest, he could not consider secret intelligence evidence relating to Russia's involvement and whether the murder could have been prevented. He said that would render his verdict "incomplete, potentially misleading and/or unfair".

Mrs Litvinenko said: "Throughout this process, from the time of Sasha's murder, I have always felt supported by the government and believed that they had a huge interest in solving this case. But now I feel the British government is only trying to protect itself. I feel I am alone.

"But it is not just in my interest to get to the truth of this. It is in the public interest. It's for everyone in Britain to ask: why? Who did it? Who brought in those radioactive materials."

She has been given until Monday afternoon to decide whether or not to continue with her bid for a judicial review. "It would be an enormous risk, which is why we need to appeal to the British public." her lawyer Elena Tsirlina of Blokh Solicitors said.

Alex Goldfarb, who is raising funds for the legal bills through the Litvinenko Justice Foundation, said: "The government is covering up for murderers who committed a horrendous crime against a British citizen on British soil. Marina is the only one who is able to stand up for the pride of the British nation."